Color photographic systems are based either on additive or subtractive principles. Additive systems are simple and inexpensive since only a single photographic emulsion is used. They are low in optical efficiency because they use deep absorbing filters to analyze the image light into the three primary colors. These filters can absorb up to 90% of the available light. Again in viewing the finished image, as by projection, a similar amount of light is lost, resulting in a very dim picture. In subtractive systems, these large light losses do not exist and today, with one exception, all commercial systems are of the subtractive type. While the light utilization efficiency is superior in subtractive systems, these use three highly critical emulsions, and often require elaborate and complex chemistry in processing. They are therefore relatively expensive.
One well-known type of additive system is the so-called "Joly" screen plate system in which the surface of a photographic film base is covered with a fine mosaic of tri-colored filter elements upon which a silver halide emulsion is coated, typically a reversal-type film.
This invention, in a preferred execution, utilizes diffraction and dispersion phenomena. Diffraction phenomena has for decades offered the hope (as yet unrealized) of a simple, low cost color photographic system with the efficiency of monochrome systems (since no color filters need be used in either the taking or viewing processes) and with the processing simplicity and low cost of monochrome systems.
Diffraction gratings were first suggested for use in color photography by R. W. Wood in 1899. He used three superimposed gratings of different periodicity. The Wood system and other ancient diffraction-type systems are described at length in "The History Of Tri-Color Photography" by E. J. Wall, Focal Press, original published in 1925, reprinted in 1970. Later diffraction-type systems are described in patents and publications including those to Carlo Bocca (U.S. Pat. No. 2,050,417), Peter Mueller (U.S. Pat. No. 3,719,127) and William Glenn (U.S. Pat. No. 3,078,338).
In prior diffraction type systems an image is multiplied with a diffraction grating, usually by forming an image of the scene on a photosensitive emulsion against which has been placed a diffraction grating. This causes the image to modulate a spatial carrier whose frequency is that of the grating. A number of images can be additively superimposed by using gratings of different periodicity or angular orientation. To display diffraction-type recordings, the recording is placed in a projection which may have a coherent or semi-coherent light source and the separate images riding on the carriers are segregated in a space (typically termed the Fourier transform space) intermediate the projector and screen. They can be separately viewed through properly oriented and sized slits or apertures (as in the Wood system) or allowed to recombine at the viewing screen. If the images are color separations of a common scene, a full color reconstruction can be displayed by proper spatial filtering of the projected light in the Fourier transform plane and appropriate recoloring of the information by the use of Wratten filters inserted at appropriate locations in the Fourier plane. As stated by Wall at page 670 in his book ( 1925 edition) the diffraction process as it has been known in the past, "is an extremely beautiful use of the phenomenon of diffraction by gratings, but may be justly described as belonging to the laboratory practically for the results can only be seen by one person at a time, or to a very few, as the scale on which they can be thrown on a screen is limited by the great loss of light common to the use of all gratings."
RCA Corporation described in 1978 a process which it called ZOD (TM) which involves the use of three zeroth order gratings for red, blue and green in a subtractive-type system. These gratings are area-modulated and the finished print is made by superimposing the three embossed film gratings in registration and fusing the edges of the film. Upon reconstruction in a projector, the gratings diffract unwanted light outside the projection lens, only the undiffracted zeroth order light appearing on the screen. The ZOD process is useful only in making large numbers of copies since the masters for embossing the film gratings for each color must be made separately for each scene. For details see "ZOD Images: Embossable Surface-Relief Structures for Color and Black-and-White Reproduction" by M. T. Gab et al, Journal of Applied Photographic Engineering, Vol. 4, No. 2, Spring, 1978.
Another ancient process of historical interest is the prismatic dispersion processes, described by Wall in his above-identified book at page 659 et seq. In such processes an image is broken up at the image plane by a ruling. The light passing through the clear areas of the ruling is dispersed by a prismatically molded surface of diffraction grating. The spectral bands thus formed are recorded side-by-side on a photosensitive recording medium. The recording medium is usually to be developed by a reversal process and viewed in the taking instrument. These systems are extremely low in efficiency, due in large part to the great fraction of available image light absorbed by the rulings.